1. Field of the Invention
The present invention relates to a multilayer piezoelectric element and a method of producing the same, in particular a method of producing such multilayer piezoelectric ceramics where reliability has been improved.
2. Description of the Related Art
In general, electronic parts of multilayer piezoelectric ceramic are composed by making stacked piezoelectric ceramic made by alternately stacking a plurality of layers of piezoelectric ceramic green sheets printed with internal electrodes of silver (Ag) or silver-palladium (Agxe2x80x94Pd) alloy, firing the stacked piezoelectric ceramic, and coating, as shown in FIG. 11, an electrically conductive paste of silver being a main component to the sintered stacked ceramic 101 at edges thereof so as to form terminal electrodes 102a, 102b, 102c. 
In addition, as shown in FIG. 12, there is also a product exposing internal electrodes 104 alternately stacked with piezoelectric ceramic layers 103 at sides of the sintered piezoelectric ceramic 101.
Electronic parts of the above multilayer piezoelectric ceramic are probable to generate a lot of minute pores within layers of sintered piezoelectric ceramic by firing, so that moisture goes into the pores by using for a long period of time under atmosphere at high temperature and high humidity, thereby to create defects in insulation between internal electrodes.
As a measure therefor, as shown in FIG. 13, external films 105 of an organic resin are formed on surfaces of the sintered piezoelectric ceramic 1 for preventing invasion or penetration of the moisture. However, it is difficult to perfectly avoid the invasion of the moisture with the external films 105 of the organic resin, and an insulating property goes down by such as migration of silver as an electrode material accompanying with use for a long time, inevitably resulting in short circuiting.
In a case where the multilayer piezoelectric element is miniaturized or complicated in configuration, the multilayer piezoelectric element sometimes exposes at its sides the edges of the internal electrode layers. If voltage is impressed to the internal electrode layers to drive the multilayer piezoelectric element under a condition where moisture is able to penetrate the edges, a metal composing the internal electrode layers is ionized and induces a so-called migration phenomena where the metal moves between electrodes in response to an electrical field. For the internal electrode layer, because of saving costs, alloys of silver being a main component, for example, Agxe2x80x94Pd alloy are used in general. However, in the internal electrode layer comprising the alloy including Ag, migration easily occurs, and in extreme cases, metal bridges comprising Ag and the like are formed between opposite internal electrode layers. As a result, electrical short circuits are often caused between the opposite internal electrode layers by the metal bridges, probably missing reliability.
The migration is accelerated particularly under high temperature, high humidity or high electrical field. Then, improvement of moisture resistance has been designed by the under various measures.
(1) A method of coating an outer face of the multilayer piezoelectric element with an insulating material of a resin film or a glass insulating layer.
(2) A method of coating the outer face of the multilayer piezoelectric element with an insulating material of silicon oxide (SiO2)
For example, JP-A-61-15382 discloses a technology which uniformly forms the silica insulating material on the exposed parts of the internal electrode layers through a decompression CVD method.
(3) A method of forming a glass insulating layer on the outer face of the multilayer piezoelectric element by a dry or wet transcribing method.
For example, JP-A-7-7193 discloses a technology which forms the glass insulating paste on a dry transcribing paper or a wet transcribing paper, delaminates, then sticks the glass insulating paste to four sides of the multilayer piezoelectric element, and heats it for coating the glass insulating material.
(4) A still further method of selectively coating the insulating material of inorganic material or high polymer material on only parts exposing the internal electrode layer of the multilayer piezoelectric element.
For example, JP-A-7-176802 discloses a technology which selectively adheres particles of inorganic materials as piezoelectric ceramic or high polymer such as polyimide onto only parts exposing the internal electrode layer of the multilayer piezoelectric element by the electrophoretic deposition so as to form the coating thereon.
(5) In the structure formed with holes in the multilayer piezoelectric element where the holes are filled with fillers, the coating is formed to the part exposing the edge of the internal electrode layer inside of the holes.
For example, JP-A-10-136665 discloses a structure formed with holes in the multilayer piezoelectric element where the holes are filled with soft fillers as silicone resin or urethane resin, and the coating is formed to the part exposing the edge of the internal electrode layer of the holes, thereby to improve the moisture resistance.
However, although depending on the above measures, the reliability of the multilayer piezoelectric element is not yet perfectly improved in view of the points mentioned below.
According to the above measure (1), if the multilayer piezoelectric element is coated on its outer face with a resin film, an effect of imparting the moisture resistance is poor, and a problem is left in there liability. In case the multilayer piezoelectric element is coated on its outer face with the glass insulating layer by an ordinary method, the moisture resistance is improved, but if elastic modulus is large different between the glass insulating layer and the piezoelectric ceramic when the multilayer piezoelectric element is driven, the glass insulating layer probably hampers displacement of the piezoelectric ceramic.
According to the above measure (2), the moisture resistance is improved, but when the multilayer piezoelectric element is driven so that tensile stress repeatedly acts on a silica film, the silica film is easily cracked. When cracking appears in the silica film, there is involved a problem that the edge of the internal electrode layer is again exposed, resulting to spoil the reliability.
For forming the silica film in a predetermined film thickness, a relatively long processing time lead to increase of the cost.
According to the above measure (3), if the outer face of the multilayer piezoelectric element is coated with the glass insulating layer, the moisture resistance is heightened, and appearing rate of defects is reduced when using it at the high humidity, but when, similarly to the measure (1), the elastic modulus is large different between the glass insulating layer and the piezoelectric ceramic, the glass insulating layer probably hampers displacement of the piezoelectric ceramic.
According to the measure (4), the moisture resistance is increased, and since the insulating material is selectively coated to only the part exposing the internal electrode layer, when the multilayer piezoelectric element is driven and the tensile stress is repeatedly acted on the inorganic coating layer, appearance of cracks can be checked. The reliability is therefore largely increased. Further, according to the electrophoretic deposition, comparing with the measure (2), the coating time of the insulating material is shortened, enabling to suppress increase of the cost more.
However, since the insulating material is formed to only parts exposing the internal electrode layers, the moisture resistance of the piezoelectric ceramic layer is not enough at parts not formed with the insulating material, for example, between the internal electrode layers, and in particular when pores exist in the piezoelectric ceramic layer, probability of spoiling the reliability is present.
According to the measure (5), the moisture resistance is improved in the part exposed at the holes, but at parts exposing the internal electrode layers outside, another coating layer should be separately provided.
In view of the above-mentioned problems, it is an object of the invention to offer electronic parts of the multilayer piezoelectric ceramic enabling to compose as scarcely causing deterioration of insulation resistance in spite of using for longer time under atmosphere at high temperature and at high humidity as well as a production method thereof.
It is another object of the invention to offer electronic parts of the multilayer piezoelectric ceramic maintaining a displacing characteristic well conditioned and enabling to compose as scarcely causing deterioration of insulation resistance as well as a production method thereof.
Further, it is an object of the invention to offer a method of producing a multilayer piezoelectric element having excellent reliability by uniformly forming the glass insulating layer at side parts exposing edges of the internal electrode layers of the multilayer piezoelectric element suppressing defects, and products thereof.
Herein, a term of xe2x80x9cglass insulating layerxe2x80x9d is defined by a coated layer comprising an amorphous inorganic compound and completely checking defects for preventing penetration of moisture.
In the electronic parts of the multilayer piezoelectric ceramic according to a first aspect of the invention, the sintered piezoelectric ceramic comprising a plurality of alternately stacked layers of internal electrodes and piezoelectric layers is a main part, provided at edges thereof with terminal electrodes conducting electricity to the internal electrodes, and is provided on surfaces thereof with films of glass insulating material.
In the electronic parts of the multilayer piezoelectric ceramic according to a second aspect of the invention, the sintered piezoelectric ceramic is provided on the surfaces thereof with films of the glass insulating material comprising any two or more components of ceramic compounds selected from elements composing all or a part of lead oxide (PbO), silicon oxide (SiO2), aluminum oxide (Al2O3) and the sintered piezoelectric ceramic.
In the electronic parts of the multilayer piezoelectric ceramic according to a third aspect of the invention, the sintered piezoelectric ceramic is provided on the surfaces thereof with the film of the glass insulating material having thickness of 0.5 to 7.0 xcexcm.
In the method of producing electronic parts of the multilayer piezoelectric ceramic according to a fourth aspect of the invention, the method comprises mixing piezoelectric ceramic powder in a binder solution so as to make slurry, making piezoelectric ceramic green sheets from the slurry, making a multilayer piezoelectric ceramic by alternately stacking a plurality of layers of the piezoelectric ceramic green sheets and internal electrodes, sintering the multilayer piezoelectric ceramic layers, followed by forming at edges of the sintered ceramic terminal electrodes electrically conducting to internal electrodes of the sintered piezoelectric ceramic, sintering the multilayer piezoelectric ceramics, followed by coating glass insulating paste on the surfaces of the sintered piezoelectric ceramic, and heat-treating the paste for making films with the glass insulating material.
In the method of producing electronic parts of the multilayer piezoelectric ceramic according to a fifth aspect of the invention, the sintered piezoelectric ceramic is coated on the surfaces thereof with the glass insulating paste comprising any two or more components of ceramic compounds selected from elements composing all or a part of lead oxide (PbO), silicon oxide (SiO2), aluminum oxide (Al2O3) and the sintered piezoelectric ceramics.
In the method of producing electronic parts of the multilayer piezoelectric ceramic according to a sixth aspect of the invention, the sintered piezoelectric ceramic is coated on the surfaces thereof with the film of the glass insulating material in thickness of 0.5 to 7.0 xcexcm.
Further, inventors have found the following constitution for accomplishing the above object so as to solve the problems.
In a method of producing a multilayer piezoelectric element which has a structure having an arrangement of a plurality of alternate stacking layers of piezoelectric ceramic layers and internal electrode layers and exposing the edges of the internal electrode layers, a seventh aspect of the invention comprises the steps of electrodepositing particles comprising a glass insulating material to the exposed sides through an electrophoretic deposition; and then carrying out a heat treatment at a predetermined temperature, thereby forming the glass insulating layer of the glass insulating material to the exposed parts only or both of the exposed parts and the surface of the piezoelectric layers of the internal electrode layers.
A eighth aspect of the invention is preferable in that the glass insulating layer has a thickness of 0.3 to 10 xcexcm in the seventh aspect.
A ninth aspect of the invention is preferable in that, in the seventh or eighth aspect, the heat-treating temperature of the glass insulating layer is lower than a firing temperature of the multilayer piezoelectric element, and the glass insulating layer is composed of any one selected from the components of the glass component which is a single one, the glass component which is a plurality of mixed substances, the single component which is dispersed with piezoelectric ceramic in the glass matrix thereof, and the plurality of mixed component which is dispersed with the piezoelectric ceramic in the glass matrix thereof.
The softening points of these glass insulating materials are preferably 500 to 900xc2x0 C.
A tenth aspect of the invention is convenient in that, in the ninth aspect, when the piezoelectric ceramic is contained in the glass insulating layer, this piezoelectric ceramic is the same as the piezoelectric ceramic composing the multilayer piezoelectric element.
An eleventh aspect of the invention is structured such that, in any one of the seventh to the ninth aspects, the glass insulating layer may contain PbO: 10 to 80 wt %, SiO2: 10 to 80 wt %, Al2O3: 0 to 50 wt %, and one of the same piezoelectric ceramic as the piezoelectric ceramic of the multilayer piezoelectric element: 0 to 50 wt %.
A twelfth aspect of the invention is that the multilayer piezoelectric element as set forth in the seventh to eleventh aspects may be for a piezoelectric actuator.
A thirteenth aspect of the invention is the multilayer piezoelectric element produced by the seventh to eleventh aspects.